Thermal processes are used for a broad variety of purposes in the food, chemical, pharmaceutical, and paper and pulp industries. In the food processing industry, such processes include the cooking of canned products such as soup and the pasteurization of dairy products such as milk and eggs. At present, more than seventy thousand thermal processing lines approved by the Food and Drug Administration exist. Because of product changes, five to ten thousand processing lines must be evaluated each year. This procedure is time consuming, expensive, and often inaccurate.
In many industries, it is desirable to know the thermal history of particulates suspended in a liquid as they travel through a thermal processing apparatus. Due to the complex nature of these systems, however, true thermal histories cannot be obtained.
Procedures for use of the Equivalent Point Method (EPM) for analyzing the thermal effects on products during continuous flow heating have been disclosed. See K. Swartzel, 47 J. Food Sci. 1886 (1982); K. Swartzel, 34 J. Agric. Food Chem. 397 (1986). The Equivalent Point Method differs substantially from previous methods in that all other methods define the thermal treatment based upon a single factor such as enzyme inactivation, microbial destruction, protein denaturation, nutrient loss, etc. The problem with these other methods is that a physical and/or chemical effect (flavor, color, product separation and gelation during storage if thermally related) may actually be the shelf life limiting factor.
Calibration materials useful in the equivalent point method are discussed in F. Sadeghi and K. Swartzel, Calibration Materials for Thermal Systems (Institute of Food Technologists 46th Annual Meeting Food Expo, Jun. 15-18, 1986) (tape available from Institute of Food Technologists). Candidate calibration materials mentioned are esters, ketones, peroxides, sugars, vitamins, enzymes and dyes.
Solid-state radiation detectors have been suggested for temperature recording. Such devices are made of a silicon substrate with a layer of lithium diffused into its surface. T. Hirsch, Whither microengineering, Chemtech 118, 121 (February 1986), suggests measuring surface capacitance or resistance in such devices to determine the degree of temperature-dependent lithium migration. G. Haugen and G. Hieftje, An Interdisciplinary Approach to Microinstrumentation, 60 Analytical Chemistry 23A, 27A (1988), suggest that, if several combinations of semiconductor and dopant are used that possess a range of activation energies in such a device, the combination of readouts corresponds to a set of integral equations that can be inverted mathematically (inverse LaPlace transform) to produce the full curve of time versus temperature.
The disadvantages of existing technology for thermal system evaluation pose severe problems for the Food Processing Industry. There is presently a critical need to accurately characterize thermal behavior within a food particle as it is pumped through a continuous flow thermal processing apparatus. The present invention is based on our continued research in this area.